The present invention relates to a new process for the production of organic carbodiimides and to the use of these carbodiimides for stabilizing plastics against hydrolysis.
Carbodiimides can be readily produced with catalysts from the phospholine oxide series by the basic process disclosed in U.S. Pat. No. 2,853,473.
This ability to catalyze easily is very desirable because it makes it possible to carry out the carbodiimidization reaction under moderate conditions. No process which ensures effective and perfect separation of the phospholine oxide catalyst from the end products of the carbodiimidization process is disclosed in the prior art.
There is a commercial need for storable carbodiimides which serve as hydrolysis stabilizers and do not cause troublesome secondary reactions, even at relatively high temperatures caused by rest activity of the catalytic material. This catalytic material cannot be removed simply by distillation. For example, in the production of distillable carbodiimides such as 2,6,2'-,6'-tetraisopropyl diphenyl carbodiimide from 2,6-diisopropylphenyl isocyanate with a technical mixture of 1-methyl-1-phospha-2-cyclopentene-1-oxide and 1-methyl-1-phospha-3-cyclopentene-1-oxide as catalyst, the catalyst cannot be removed completely by simple distillation. The product therefore remains "residually active", even when a relatively large head fraction is removed and even where carrier gases, such as nitrogen or argon, are present.
German Auslegeschrift 1 156 401 (U.S. Pat. No. 3,502,722) is directed to the production of storage stable carbodiimides using KOH, for example, as the catalyst to prevent troublesome residual activities. By comparison with phospholine oxides as catalysts, however, this process has serious disadvantages. Large quantities of catalyst and rigorous reaction conditions are required and technically inferior colored end products are obtained. This process cannot be adopted for non-distillable (poly)carbodiimides.